Heretofore a variety of curable resin compositions are known including those comprising hydroxyl group containing acrylic or polyester polymers and a crosslinker such as melamine resins or organic polyisocyanates. The melamine resin-crosslinking compositions require relatively high curing temperatures as high as 150.degree.-230.degree. C. This makes the composition less workable and uneconomical in use. Besides the melamine resin-crosslinking system must suffer not only from volumetric shrinkage upon curing due to the emanation of lower alkanol by-products but also from a decreased weatherability due to the presence of triazine nuclei. The polyisocyanate-crosslinking system has problems of toxicity and decreasing weatherability with time.
Japanese Laid Open (Kokai) Patent Application Nos. 36109/82 and 155660/83 disclose a system having an enhanced weatherability comprising room temperature-curable alkoxysilyl group-bearing acrylic polymers. This system has certain defects that the curing reaction proceeds slowly, that the cured product can be blotted with by-products produced by the hydrolysis of residual alkoxysilyl groups and that its rust prevention property is not satisfactory.
Japanese Patent Publication (Kokoku) No. 33152/88 discloses a thermosetting resin system utilizing a reaction between an alkoxysilyl component and a hydroxyl group-containing resin component. This system is also disadvantageous in that its curing reaction proceeds slowly and the resulting cured film exhibits a poor salt spray resistance.
It has long been known in the art to crosslink vinylpolysiloxanes with organohydrogenpolysiloxanes in the presence of a platinum catalyst. Unfortunately, the cured product of this composition is swellable with conventional organic solvents and thus exhibits poor solvent resistance as well as poor alkali resistance. This composition also has a poor overlaying property when used for coating purposes. These defects have precluded the composition from its application to coating compositions of general use. Attempts have been made to improve the overlaying property of vinylpolysiloxane based compositions by incorporating propylalkoxysiloxane-modified polyester (e.g. Japanese Laid Open Patent Application No. 263265/87) or using alkoxysilylalkylpolysiloxanes as a crosslinking agent (e.g. Japanese Laid Open Patent Application No. 127733/86). Even these attempts, a composition satisfying with mechanical strength properties, compatibility with other resins, overlaying properties etc., has not been discovered yet.
Furthermore, emission of organic solvents from coating compositions to the atmosphere has become a current problem for environmental reasons.
We have found that when crosslinking unsaturated organic polymers utilizing their addition reaction with organo-hydrogenpolysiloxanes, the resulting film may be greatly improved in various properties including weatherability, solvent, alkali resistance, acid resistance, waterproofness, salt spray resistance, rust preventive property, anti-blotting property, overlaying property and so on. We have also found that when using the above composition for coating purposes, the use of organic solvents may be dispensed with. The present invention has its basis on these findings.
Accordingly, it is an object of this invention to provide a curable resin composition which is excellent in weatherability, acid resistance, alkali resistance, waterproofness, salt spray resistance, solvent resistance, overlaying property and so on.
It is another object of this invention to provide a curable resin composition which is curable at relatively low temperatures without volumetric shrinkage and thus may also be used for molding purposes.
It is a further object of this invention to provide a curable resin composition which may be used for coating purposes as solventless or high solids coating compositions.